Mitt Romney vowed Wednesday to expand Washington’s school voucher program as part of a broader nationwide push for school choice, and he accused President Obama of failing to fulfill his own education promises from 2008 because he is too beholden to teachers unions.

Opening a new line of attack on the president, Mr. Romney, the presumptive Republican nominee, said unions are the chief impediment to education reform, and that Mr. Obama has repeatedly sided with them instead of with parents and students attending failing schools.

The attack was part of an education speech Mr. Romney delivered in the nation’s capital to the Latino Coalition, a Hispanic small-business advocacy group, and signaled an astute political calculation: Hispanic voters regularly place education among their top issues, even higher than immigration, and they generally support vouchers and stricter school standards.

“Here we are in the most prosperous nation, but millions of children are getting a Third World education. And America’s minority children suffer the most,” Mr. Romney said. “This is the civil rights issue of our era. And it’s the great challenge of our time.”

Mr. Romney’s chief reform would be to give children who receive federal education money a choice of any public or charter school in their state or, in cases where it’s legal under state law, private schools. He also said he’ll push for more usable evaluations of schools so parents have the information they need to make choices, and said he’ll streamline federal teacher quality programs to reward states that are doing best at training and retaining good teachers.

In the most pointed part of his plan, Mr. Romney said he would fight the teachers unions, which he called “the clearest example of a group that has lost its way.” He said teachers unions have teamed up with Democrats to block reforms that have shown promise in Connecticut, Detroit and in the District, where the city’s voucher program has proved wildly popular with parents, but saw Democrats try to cut it.

“In the Opportunity Scholarships, the Democrats finally found the one federal program they are willing to cut. Why? Because success anywhere in our public schools is a rebuke to failure everywhere else,” Mr. Romney said. “That’s why the unions oppose even the most common-sense improvements.”

Driving his message home, he plans to visit a charter school in Philadelphia on Thursday.

The Obama campaign said voters should be wary of Mr. Romney’s promises, saying he cut education funding when he was governor of Massachusetts and has backed budget plans that would call for cuts to domestic spending, including education.

The National Education Association, the country’s largest teachers union, said Mr. Romney’s plans were a rehash of President George W. Bush’s education policy. The association also criticized the candidate’s list of education advisers, saying it was stacked with opponents of public schools.

The NEA endorsed Mr. Obama for re-election last July, well before the Republican nomination had been settled.

Both the White House and the NEA mocked Mr. Romney for speaking out now on education, saying if he cared about the issue he should have made it a bigger part of his primary election campaign.

Mr. Romney said if it weren’t for the economy, education would be the most important topic in this year’s election.

A number of Republican governors have won office in recent years on promises of education reforms, and highlighting education before a Hispanic audience could help Mr. Romney make inroads with that important group of voters.

Raul Gonzalez, director of legislative affairs for the National Council of La Raza, the largest umbrella organization for Hispanic groups, said Hispanics do consider education to be a major civil rights issue and that Mr. Romney’s push for vouchers likely will play well.

Still, he said, other than Mr. Romney’s push for school choice, and Mr. Obama’s Race to the Top program that rewards innovative schools, the two men share many similarities on education. Both favor moving away from some of the accountability provisions in Mr. Bush’s No Child Left Behind legislation, and have stressed teacher effectiveness as a solution for poor schools.

Mr. Gonzalez said there’s plenty of room for both men to have a more detailed discussion of education. He said he will be looking to see whether Mr. Romney pushes reforms that make it easier for communities to start charter schools.

Possibly the biggest difference between Mr. Obama and Mr. Romney is on the District’s Opportunity Scholarship Program, which offers federal taxpayer-funded vouchers to low-income students.

It has been contentious from the time it started a decade ago, with the powerful teachers unions opposing it but city parents in support.

As of this year, more than 1,600 students are enrolled in the program, which offers scholarships of up to $8,000 through eighth grade and $12,000 for ninth through 12th grades. The money goes to pay tuition at private schools, including religiously affiliated schools.

Mr. Obama tried to end the program when he took office, but eventually reached a compromise that let students already in the program continue, but halted new applications.

When Republicans took control of the House last year, Speaker John A. Boehner fought to restart the program, and insisted that funding be included in spending bills.

But the program remains on edge, and Mr. Obama’s 2013 budget, submitted in January, doesn’t include any money for it going forward. His administration argues that it has enough money to cover students for the next year.

Mr. Boehner, Ohio Republican, and Sen. Joe Lieberman, Connecticut independent, questioned Mr. Obama’s decision, and Mr. Boehner has vowed to demand that the program be funded.